1. Field of the Invention
The present invention relates to an oxide magnetic material having soft magnetism, and more specifically to a Mnxe2x80x94Zn ferrite suitable for use as a switching power transformer, a rotary transformer and the like, and to a production process thereof.
2. Description of the Related Art
Typical oxide magnetic materials having soft magnetism include a Mnxe2x80x94Zn ferrite. Conventionally, this Mnxe2x80x94Zn ferrite usually has a basic component composition containing 52 to 55 mol % Fe2O3on the average, exceeding 50 mol % which is the stoichiometric composition, 10 to 24 mol % ZnO and remainder MnO. The Mnxe2x80x94Zn ferrite is usually produced by mixing respective material powders of Fe2O3, ZnO and Mno in a prescribed ratio, subjecting the mixed powders to respective steps of calcination, milling, component adjustment, granulation and pressing to obtain a desired shape, then performing sintering treatment at 1200 to 1400xc2x0 C. for 2 to 4 hours in a reducing atmosphere in which a relative partial pressure of oxygen is controlled to a low level by supplying nitrogen. The Mnxe2x80x94Zn ferrite is sintered in the reducing atmosphere in order to reduce a part of Fe3+ thereby forming Fe2+. This Fe2+ has positive crystal magnetic anisotropy and cancels negative crystal magnetic anisotropy of Fe3 to thereby enhance soft magnetism.
Amount of the above-mentioned Fe2+ formed depends on relative partial pressures of oxygen in sintering and cooling after the sintering. Therefore, when the relative partial pressure of oxygen is improperly set, it becomes difficult to ensure excellent soft magnetic properties. Thus, conventionally, the following expression (1) has been experimentally established and the relative partial pressure of oxygen in sintering and in cooling after the sintering has been strictly controlled in accordance with this expression (1).
log Po2=xe2x88x9214540/(T+273)+bxe2x80x83xe2x80x83(1)
where T is temperature (xc2x0 C.), Po2 is a relative partial pressure of oxygen, and b is a constant which is usually 7 to 8. The fact that the constant b is set to 7 to 8 means that the relative partial pressure of oxygen in the sintering must be controlled in a narrow range, whereby such a problem arises that the sintering treatment becomes significantly troublesome and therefore production costs are increased.
In recent years, with miniaturization and performance improvement of electronic equipments there is such an increasing tendency that signals are processed at a higher frequency. Thus, a magnetic material having excellent magnetic properties even in a higher frequency region as well has been needed
However, when the Mnxe2x80x94Zn ferrite is used as a magnetic core material, an eddy current flows in a higher frequency region applied resulting in a larger loss. Therefore, in order to extend an upper limit of the frequency at which the Mnxe2x80x94Zn ferrite can be applied as a magnetic core material, an electrical resistivity of the material must be made as high as possible. However, since the above-mentioned general Mnxe2x80x94Zn ferrite contains Fe2O3 in an amount larger than 50 mol % which is the stoichiometric composition, a large amount of Fe2+ ion is present, thereby making easy the transfer of electrons between the above-mentioned Fe3+ and Fe2+ ions. Thus, the electrical resistivity of the Mnxe2x80x94Zn ferrite is in the order of 1 xcexa9m or less. Accordingly, an applicable frequency is limited to about several hundred kHz maximum, and in a frequency region exceeding the limit, permeability (initial permeability) is significantly lowered to completely take away properties of the soft magnetic material.
In order to increase an apparent resistance of the Mnxe2x80x94Zn ferrite, in some cases, CaO, SiO2 or the like is added as additive to impart a higher resistance to grain boundary and at the same time the Mnxe2x80x94Zn ferrite is sintered at as low as about 1200xc2x0 C. to diminish the grain size from its usual dimension, about 20 xcexcm, to 5 xcexcm, thereby taking measures to increase the ratio of the grain boundary. However, even if such measures are adopted, it is difficult to obtain an electrical resistivity exceeding 1 xcexa9m order as the grain boundary itself has a low resistance, and the above-mentioned measures fall short of a thorough solution.
Further, a Mnxe2x80x94Zn ferrite to which, for example, CaO, SiO2, SnO2 and TiO2 are added to obtain a higher resistivity has been developed and is disclosed in Japanese Patent Application No. Hei 9-180925. However, the electrical resistivity of the Mnxe2x80x94Zn ferrite is as low as 0.3 to 2.0 xcexa9m, which does not sufficiently satisfy application in a high frequency region. Further, a Mnxe2x80x94Zn ferrite to which SnO2 and the like are added is disclosed in EPC 1,304,237. The Mnxe2x80x94Zn ferrite described in this EPC patent contains as much as 3 to 7 mol % Fe2+. An electrical resistivity depends on amount of Fe2+ as described above. Therefore, the electrical resistivities of the Mnxe2x80x94Zn ferrite in this EPC patent cannot exceed the electrical resistivities of a usual Mnxe2x80x94Zn ferrite of the prior art.
On the other hand, a Mnxe2x80x94Zn ferrite which contains less than 50 mol % Fe2O3 for a higher resistance has been developed for use as a core material for a deflecting yoke and is disclosed in Japanese Patent Laid-open Nos. Hei 7-230909, Hei 10-208926, Hei 1-99235 and the like.
However, judging from the fact that the application thereof is a core material for a deflecting yoke and from examples of the invention described in each publication, the Mnxe2x80x94Zn ferrites described in any of the above publications are ferrite materials intended to be used in a frequency region of 64 to 100 kHz. It is described that the purpose in setting Fe2O3 content to 50 mol % or less for a high electrical resistivity is to enable a copper wire to be wound directly around a core for a deflecting yoke. Excellent magnetic properties are not obtained in such a high frequency region as exceeding 1 MHz. Thus, only setting the Fe2O3 content to less than 50 mol % for a high electrical resistivity is not good enough to enable the ferrites to be used as a magnetic core material in such a high frequency region as exceeding 1 MHz.
The present invention has been made in consideration of the above-mentioned conventional problems. An object of the present invention is to provide a Mnxe2x80x94Zn ferrite that has, of course, excellent magnetic properties, and also has both a higher electrical resistivity than 1 xcexa9m order (a single digit order) and a low core loss in such a high frequency region as exceeding 1 MHz, and a production process by which such a Mnxe2x80x94Zn ferrite can be obtained easily and inexpensively.
A Mnxe2x80x94Zn ferrite according to the present invention to attain the above-mentioned object is characterized in that its basic component composition includes 44.0 to 49.8 mol % Fe2O3, 6.0 to 15.0 mol % ZnO (15.0 mol % is excluded), 0.1 to 4.0 mol % at least one of TiO2 and SnO2, and remainder MnO, and that the average grain size is less than 10 xcexcm.
Another Mnxe2x80x94Zn ferrite according to the present invention is characterized in that its basic component composition includes 44.0 to 49.8 mol % Fe2O3, 6.0 to 15.0 mol % ZnO (15.0 mol % is excluded), 0.1 to 4.0 mol % at least one of TiO2 and SnO2, 0.1 to 6.0 mol % CuO, and remainder MnO, and that the average grain size is less than 10 xcexcm.
Still another Mnxe2x80x94Zn ferrite according to the present invention may contain as additive, in addition to the basic component compositions of the above-described two inventions, at least one component selected from the group consisting of 0.005 to 0.200 mass % CaO, 0.005 to 0.050 mass % SiO2, 0.010 to 0.200 mass % ZrO2, 0.010 to 0.200 mass % Ta2O5, 0.010 to 0.200 mass % HfO2 and 0.010 to 0.200 mass % Nb2O5.
On the other hand, a production process according to the present invention to attain the above-mentioned object is characterized in that a mixed powder whose components are adjusted so as to compose the above-mentioned Mnxe2x80x94Zn ferrite is pressed, then sintered and cooled, after the sintering, down to 500xc2x0 C. or lower in an atmosphere of a relative partial pressure of oxygen defined by an arbitrary value selected from a range of 6 to 12 as a constant b in the expression (1).
In a usual Mnxe2x80x94Zn ferrite of the prior art, Fe2O3 content is larger than 50 mol % that is the stoichiometric composition, as described above. In order to prevent this excessive Fe2O3 from getting precipitated as hematite, sintering and cooling must be conducted under a condition where a relative partial pressure of oxygen is reduced to a significantly lower level by flowing nitrogen, that is a condition where the constant b in the expression (1) is set to 7 to 8. On the other hand, since a Mnxe2x80x94Zn ferrite of the present invention contains 44.0 to 49.8 mol % Fe2O3, that is less than 50 mol %, hematite is hardly precipitated. Thus, even if a range of relative partial pressure of oxygen in sintering is somewhat increased, excellent magnetic properties can be obtained. Further, in the conventional Mnxe2x80x94Zn ferrite that contains more than 50 mol % Fe2O3, about 3.0 mol % Fe2+ exists. On the other hand, in the Mnxe2x80x94Zn ferrite of the present invention, Fe2+ content is as low as 0.1 to 0.7 mol %. Accordingly, the electrical resistivity of the Mnxe2x80x94Zn ferrite of the present invention is very high. Therefore, an eddy current is not increased so much even in a high frequency region, and excellent initial permeability can be obtained. However, if this Fe2O3 content is too small, saturation magnetization is deteriorated. Thus, at least 44.0 mol % Fe2O3 must be contained.
ZnO as main component influences the Curie temperature and saturation magnetization. Too small amount of ZnO reduces the initial permeability, but on the contrary too large amount of ZnO lowers the saturation magnetization and Curie temperature. Since ferrite for power transformer is often used in an environment at a temperature of about 80 to 100xc2x0 C., it is particularly important that the ferrite has a high Curie temperature and high saturation magnetization. Accordingly, ZnO content in the ferrite is set to the above-mentioned range of 6.0 to 15.0 mol % (15.0 mol % is excluded).
Ti and Si receive an electron from Fe3+ to thereby form Fe2+. Therefore, amount of Fe2+ formation can be inhibited by controlling TiO2 or SnO2 content and relative partial pressure of oxygen in sintering and cooling after the sintering. The existing ratio of Fe3+ to Fe2+ is optimized to cancel out positive and negative crystal magnetic anisotropies with the result that the ferrite has excellent soft magnetism. However, when the TiO2 or SnO2 content is too small, the effect is small, whereas initial permeability is lowered when the content is too large. Thus, the TiO2 or SnO2 content is set within a range of from 0.1 to 4.0 mol %.
In the present invention, CuO may be contained additionally as a main component. This CuO has an effect of enabling the ferrite to be sintered at a low temperature. However, if CuO content is too small, the effect is small. On the contrary if the CuO content is too large, core loss increases. Accordingly, the CuO content is set to 0.1 to 0.6 mol %.
In the present invention, CaO, SiO2, ZrO2, Ta2O5, HfO2 or Nb2O5 can be contained as additive. These additives have an action of accelerating crystal grain growth and are effective in keeping average grain size less than 10 xcexcm. However, if their content is too small, the effect is small, and on the contrary if the content is too large, grains grow abnormally. Thus, CaO content is set to 0.005 to 0.200 mass %, SiO2 content is set to 0.005 to 0.050 mass %, ZrO2 content is set to 0.010 to 0.200 mass %, Ta2O5 content is set to 0.010 to 0.200 mass %, HfO2 content is set to 0.010 to 0.200 mass %, and Nb2O5 content is set to 0.010 to 0.200 mass %.
The core loss of ferrite in a high frequency region comprises mainly eddy-current loss and residual loss. As described above, the Mnxe2x80x94Zn ferrite according to the present invention has a very high electrical resistivity and a small eddy-current loss. Further, since the Mnxe2x80x94Zn ferrite has a small average grain size of less than 10 xcexcm, number of magnetic domain walls in a crystal grain is decreased, whereby the residual loss can be significantly decreased.
In the present invention, as described above, the sintering and cooling after the sintering can be conducted in an atmosphere of the relative partial pressure of oxygen obtained using an arbitrary value in a range of 6 to 12 as the constant b in the expression (1). However, when a value larger than 12 is selected as the constant b, Fe2+ is hardly formed with the result that core loss increases. On the contrary, when a value smaller than 6 is selected, the electrical resistivity is significantly lowered by the fact that amount of Fe2+ increases.
In production of the Mnxe2x80x94Zn ferrite, respective raw material powders of Fe2O3, ZnO, TiO2, SnO2, CuO and MnO, which are main components, are previously weighed for a prescribed ratio and mixed to obtain a mixed powder, and then this mixed powder is calcined and finely milled. The temperature for calcination differs slightly depending on target compositions and an appropriate temperatures should be selected from a range of 800 to 1000xc2x0 C. A general-purpose ball mill can be used for the fine milling of the calcined powder. When CaO, SiO2, ZrO2, Ta2O5, HfO2 or Nb2O5 is made to be contained as additive, powders of these additives are added to the aforementioned fine milled powder in appropriate amounts and mixed with each other to obtain a mixture with a target composition. Then, the mixture is granulated and pressed in accordance with a usual ferrite production process, and then sintered at 1000 to 1400xc2x0 C. In the granulation process, a method of adding a binder such as polyvinyl alcohol, polyacrylamide, methyl cellulose, polyethylene oxide or glycerin can be used, and in the pressing process, a method of applying a pressure of, for example, 80 MPa or more can be used.
In the above-mentioned sintering and cooling after the sintering, relative partial pressure of oxygen is adjusted by flowing inert gas such as nitrogen gas or the like into a sintering furnace. In this case, an arbitrary value can be selected from a range of 6 to 12 as the constant b in the expression (1), which provides a very wide allowance as compared to the constant b (7 to 8) selected in a case where a usual Mnxe2x80x94Zn ferrite of the prior art containing more than 50 mol % Fe2O3 is sintered, and the relative partial pressure of oxygen can be easily controlled. Further, in this case, since at a temperature of below 500xc2x0 C., the reaction of oxidation or reduction can be neglected irrespective of relative partial pressure of oxygen, the cooling after the sintering needs to be conducted in accordance with the above-mentioned expression (1) only till the temperature gets down to 500xc2x0 C.